This invention relates to a method of manufacturing a glass substrate for a magnetic disk adapted to be mounted in a magnetic disk device such as a HDD (hard disk drive), and so on, and in particular, relates to a cleaning solution and a cleaning method for use in cleaning the surface of a magnetic recording disk glass substrate that is subjected to a problem of particle contamination firmly adhering to the substrate surface, and so on.
The manufacturing process of a magnetic recording disk glass substrate and a magnetic recording disk carries out thin film formation on the glass substrate surface in the order of several nm, formation of recording/reproducing tracks, and so on. Therefore, in the manufacturing process of them, it is an important subject to remove even slight contamination on the glass substrate surface to thereby keep the substrate surface clean. Among the contamination, particularly the particle contamination is required to be removed in the manufacturing process of the magnetic recording disk glass substrate because it causes roughness (irregularities) on the surface after the film formation and lowers electrical properties such as recording/reproduction and yield of products. Since the particle contamination due to abrasive residue and metal particles is produced in the manufacturing process of the magnetic recording disk glass substrate, the surface cleaning with a cleaning solution is used in order to remove such particle contamination.
Generally, it is known that an alkaline aqueous solution is effective for removing particle contamination such as particles adhering or adsorbing to the substrate surface with a relatively weak force. Specifically, use is made of an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide, or a cleaning solution added with a surfactant.
Use is also made of an acidic cleaning solution that lifts off the foreign matter adhering to the substrate surface with a relatively strong force or biting into the substrate surface (thoroughly eliminates particles along with the substrate surface) by mainly etching the substrate. As the cleaning solution for the liftoff, use is made of a fluorine-based chemical solution, for example, a dilute solution of hydrofluoric acid, hydrosilicofluoric acid, ammonium fluoride, buffered hydrofluoric acid, or the like.
Further, as a cleaning solution for dissolving and removing the foreign matter itself adhering to the substrate surface with a relatively strong force or biting into the substrate surface, use is made of a cleaning solution using an inorganic acid such as sulfuric acid or nitric acid, an organic acid such as oxalic acid or ascorbic acid, or a chelating agent such as EDTA (ethylenediaminetetraacetate) or citric acid.
Among the adhering particles, particularly the particle contamination adhering to the surface is firmly adhering or fusion-bonded to the glass surface before and after a glass strengthening process and, therefore, cannot be removed by alkaline cleaning or acid cleaning generally used for cleaning the particle contamination. As a method of cleaning the particle contamination that is firmly adhering or fusion-bonded to the glass surface before and after such a glass strengthening process and cannot be removed by the alkaline cleaning or the acid cleaning, use is made of a method (liftoff method) that entirely strips the adhering particles by thinly etching the glass substrate surface by the use of a solution containing a fluoride, for example, hydrofluoric acid, ammonium fluoride, silicofluoric acid, or the like.
When the liftoff method is used, since the substrate itself is etched, the surface roughness of the substrate itself problematically increases if the etching amount is increased. Conversely, if the etching amount is reduced so as not to increase the surface roughness, objects cannot be lifted off and problematically remain as minute projections.
The particle contamination firmly adhering or fusion-bonded to the substrate surface is often an abrasive component such as cerium oxide or foreign matter (iron-based contamination) such as iron oxide that is in a thermodynamically very stable state. It is known that cerium oxide-based particles can be dissolved and removed by a mixed solution of sulfuric acid and aqueous hydrogen peroxide, or a solution in a combination of an acid and a reducing agent such as ascorbic acid, which is described in Japanese Unexamined Patent Publication No. 2001-206737 (Patent Document 1). However, a chemical solution effective for rapidly dissolving transition metal oxide-based, particularly iron oxide-based particles themselves has not been found except very reactive heated concentrated sulfuric acid-aqueous hydrogen peroxide.